Content-Type: image/png; not visible

I'm responsible for filtering our spam queue and we get a load of image only emails although I just delete them I noticed that emails like this with Content-Type: image/png; do not have an attachment shown (there is an paper clip in the list view) whereas Content-Type: image/jpg; are shown. Not a problem until we get a legitimate image only email with an Content-Type: image/png. In the png email Zimbra seems to know an attachment is there because of the paper clip and you get the "Attachments:" label under the "To:" but no attachment name, size, download or add to briefcase is displayed. Censored Headers;